Lash adjusters have been put into practical use for automatically adjusting a valve clearance provided between a cylinder head and intake/exhaust valves of an internal combustion engine to substantially zero. With such lash adjusters, clatter noises generated by interference between the cylinder head and the intake/exhaust valves can be eliminated from a functional standpoint, and work to periodically inspect and adjust the valve clearance is no longer necessary from a maintenance standpoint. There is a type of lash adjuster that is externally supplied with oil and utilizes engine oil, for example. In the case of the lash adjuster externally supplied with oil, an unsuitable engine oil injection amount or the use of deteriorated oil increases the risk of air and foreign matter becoming mixed in the oil, and the function of the lash adjuster may be compromised due to these factors. In other words, the function of the lash adjuster externally supplied with oil is dependent on the quality of engine oil maintenance. On the other hand, there is also a sealed type of lash adjuster in which fluid such as oil or the like is sealed within the lash adjuster. With such a sealed lash adjuster, the above-described factors interfering with lash adjuster function can be eliminated.
With regard to sealed lash adjusters, Patent Document 1, for example, proposes a sealed lash adjuster as described below. The sealed lash adjuster proposed in Patent Document 1 provides an oil seal having an X-shaped cross section with two lips (simply called an “X-seal” below) between a body and a plunger. The X-seal can simultaneously seal two fluids on both sides of the seal. Therefore, the sealed lash adjuster has a configuration for simultaneously preventing oil sealed by the X-seal (simply called “enclosed oil” below) from flowing outside and preventing external fluid such as engine oil (simply called “external oil” below) from flowing inside. Note that, other art that can be considered related to the present invention is proposed in Patent Documents 2 to 4, for example.    Patent Document 1: Japanese Patent Publication No. 62-29605    Patent Document 2: Japanese Patent Application Publication No. 2005-162138    Patent Document 3: Japanese Patent Application Publication No. 2006-132660    Patent Document 4: Japanese Utility Model Application Publication No. 01-124008